In refrigerating systems as used in motor vehicles, for example in refrigerated vehicles or in the air-conditioning systems of omnibuses or automobiles, the compressor is generally driven by the internal combustion engine provided for driving the motor vehicle. With these internal combustion engines in travelling operation very different speeds occur which can lie in a range of for example 450 r.p.m. to 10000 r.p.m. To ensure that the refrigerating power of the refrigerating system remains substantially constant, however, the speed of the compressor should vary as little as possible. A usual nominal speed of the compressor is for example 2400 r.p.m. The changes in the speeds are particularly great in the case of internal combustion engines of omnibuses travelling from one bus stop to the next so that in them the speed varies continuously over the entire range. In this case in the prior art it is hardly possible to design an air-conditioning system which is optimally constructed for the air-conditioning of the bus. In the case of refrigerated vehicles in the prior art even a separate drive motor running with a constant speed is used for the drive of the compressor because a constant compressor speed cannot be achieved via the internal combustion engine provided for the drive of the refrigerated vehicle. However, the use of a separate drive motor for the compressor is expensive and uneconomical because the vehicle must carry an additional motor whose additional weight decreases the efficiency of the vehicle. Usually, such a separate drive motor for the compressor is also an internal combustion engine. Alternatively, however, electrical generators have already been used which are coupled to the drive internal combustion engine and supply an electric motor with current via an appropriate control in such a manner that said electric motor drives the compressor with a constant speed. This known arrangement for influencing the speed of a compressor of a refrigerating system is however even more expensive than that previously described because it involves more outlay and because the electrical generator must be designed for the entire range of the speed of the internal combustion engine, i.e. must practically be considerably overdimensioned.
To avoid these disadvantages of the arrangements described for influencing the speed of rotation of a compressor in a further known arrangement a magnetic slip coupling is simply used which couples one of the V-belt pulleys of the V-belt drive of the compressor to the associated shaft of the internal combustion engine provided for the drive of the vehicle or of the compressor. The V-belt drive has a fixed transmission ratio, the speed of the internal combustion engine is measured and from a certain nominal speed of the compressor onwards the latter is uncoupled by the magnetic coupling. The magnetic coupling then operates from the nominal speed onwards with slip and as a result high thermal stresses occur in the magnetic coupling and frequently lead to failure of such magnetic couplings. A further disadvantage is that the compressor is driven beneath the nominal speed with the variable speed of the internal combustion engine, i.e. cannot provide a constant delivery. Operation of the compressor with a speed below its nominal speed involves a further disadvantage because with decreasing compressor speed less refrigerant is taken from the evaporator by the compressor and in the extreme case this can lead to the evaporator being flooded, i.e. filling with liquid refrigerant. Since liquid is not compressible to avoid destruction of the compressor the latter must be made substantially larger than really necessary. This involves further additional weight, which is fundamentally unfavourable in motor vehicles.
Moreover, these known arrangements influence the speed of the compressor of the refrigerating system only in dependence upon the speed of the drive internal combustion engine. It is assumed that a certain compressor speed corresponds to a certain refigerating power and consequently the latter can be kept constant by keeping the compressor speed constant. This simplifying assumption disregards that the pressure within the evaporator also greatly depends on the ambient temperature. The fact that with each ambient temperature a quite definite refrigerant pressure in the evaporator is associated is not taken into account in the known arrangements when the latter influence the compressor speed only in dependence upon the drive internal combustion engine speed.
In a known arrangement of the type set forth in the preamble of claim 1 (DE No. 27 38 728 Al) the refrigerating power of a refrigerating compressor is to be controlled in order to better utilize the power capacity of the refrigerating compressor with decreasing temperatures with the aim of a constant mass throughput. This known control arrangement is based on the knowledge that when with decreasing temperatures the required refrigerating power decreases said refrigerating power of the refrigerating compressor can be controlled via the speed of the electric motor driving it with the aid of the control arrangement in such a manner that with decreasing temperature and decreasing mass throughput of the refrigerant per stroke by increasing the speed of the electric motor the mass throughput can be increased again. However, in this manner the refrigerating power of the refrigerating system is not regulatable because with further decreasing temperature of the refrigerant in the evaporator the energy content of said refigerant also decreases and certainly cannot be increased by increasing the mass throughput. Consequently, this known arrangement is at the most suitable for a heat pump but not for the refrigerating system of an air-conditioning system. Morevoer, this known arrangement requires a drive motor whose speed must be influenced for the control. There would be no control possibility if the speed of the drive motor changed arbitrarily.